President Barack Obama and rival Mitt Romney paused their campaigns Tuesday to observe the 11th anniversary of the terrorist attacks of Sept. 11, 2001.

Another significant anniversary comes later this week — the fourth anniversary of the bankruptcy filing of Lehman Brothers.

Osama bin Laden hoped to destabilize the U.S. financial system by attacking the World Trade Center towers near Wall Street. The system was shaken but withstood the assault.

But the financial system and the larger U.S. economy are still reeling from the collapse of Lehman and other financial giants.

Failing to get a government bailout, Lehman sought bankruptcy protection on Sept. 15, 2008.

Banks stopped lending to each other, credit froze and the Dow dropped 500 points, accelerating the worst downturn since the Great Depression. Banks and the larger economy are slowly coming back, but unemployment remains above 8 percent and growth painfully slow.

The economy is the top U.S. campaign issue, while recessions in Europe and a sharp slowdown in China further threaten the recovery.

America's presidential election and China's leadership transition are happening at roughly the same time.

The U.S. race is close to a dead heat. And the strange disappearance of China's leader-in-waiting — Xi Jinping dropped from sight 10 days ago — means that politics in the world's two biggest economies right now remains unpredictable.

Obama participated in a series of 9/11 observances, recalling a day "when grief crashed over us like an awful wave."

Romney shook hands with firefighters in Chicago and was speaking to a National Guard convention in Nevada. "America shall remain ever vigilant against those who would do us harm," he said in a statement.

Vice President Joe Biden spoke at a memorial service in Shanksville, Pa. His GOP counterpart, Rep. Paul Ryan was in his home state of Wisconsin.